LED Resistor Calculator

Supply voltage, V

LED forward voltage, V

Красный около 2 В, синий и белый около 3,2 В.

LED current, mA

Number of LEDs

Wiring

Result

Nearest standard value150 Ом
Exact resistance150 Ом
Dissipation, W0.06
Resistor rating0,125 Вт хватит
Total current, mA20
Resistance, Ω150

Without a resistor an LED lasts seconds

Sizes the series resistor from supply voltage, LED forward drop and working current, and picks the nearest standard value.

An LED is a diode, not a bulb: past its forward voltage the resistance collapses and current runs away. Wired straight across a supply it destroys itself almost immediately, so the current is always limited by a resistor.

The sum is trivial: subtract the LED's forward drop from the supply and divide by the wanted current. Then take the next standard value upward, so the LED is never overdriven.

What this calculator shows

The exact resistance and the nearest standard E24 value
The power the resistor will dissipate and the rating it needs
Total circuit current when several LEDs are involved

What to keep in mind

Forward voltage depends on colour: about 2 V for red, 2.2 for green, 3.0 to 3.4 for blue and white. The datasheet has the exact figure.
20 mA is the classic indicator current; modern high-efficiency parts are bright at 5 mA, and power LEDs need a driver rather than a resistor.

FAQs

Can I run an LED without a resistor?

Only from a source that limits current itself, such as a proper driver. Straight from a battery or a supply it fails within seconds.

Series or parallel?

Series is better: the same current passes through every LED and one resistor does the job. In parallel, part-to-part variation makes one LED brighter and shorter-lived, so each needs its own resistor.

Why round the resistor up rather than down?

A larger resistor slightly reduces current and brightness; a smaller one exceeds the rated current and shortens life. Between two neighbouring values, larger is always the safe choice.

What is my LED's forward voltage?

Check the datasheet for Vf. As a guide: red and yellow around 2 V, green 2.2, blue, white and UV between 3 and 3.4.

Worked example

A red LED from USB

Input: 5 V supply, 2 V forward, 20 mA

Output: Exactly 150 Ω, standard 150 Ω, 0.06 W

Note: Tiny dissipation — the smallest 0.125 W part will do. Run the same LED from 12 volts and the resistor sheds 0.2 W, at which point a quarter-watt part becomes mandatory.

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Result

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